According to the state of the art, the procedure in reliability tests for testing apparatuses e.g. those for empty drinks bottles, so-called empty bottle inspectors, is that a row of test bottles is prepared so that each contains a certain defect according to the defect recognition specification, i.e. does not fulfil a certain feature. A special test bottle is prepared for every feature checked. The batch of test bottles is then incorporated in the stream of bottles at certain time intervals, e.g. every half hour, or after a certain number of bottles, e.g. 50,000 bottles. This operation can be automatic or manual. The test bottles are marked so that they are instantly recognisable as test bottles. The reliability test consists of checking whether these test bottles can be recognised as defective by the testing apparatus, for example the empty bottle inspector. In the method used up until now, the second condition is thus complementary to the first condition, i.e. the second condition is fulfilled in the test bottles if the test apparatus recognises that the test bottle is defective, i.e. that the first condition is not fulfilled. A record is kept in the testing apparatus. If the reliability test fails, i.e. if one or more of the test bottles is not recognised as defective, the test must be repeated. This is to ensure the operational dependability, i.e. the reliability, of a testing apparatus. This reliability test is unsatisfactory as it is only subsequently recognised that a testing apparatus has no longer functional reliably. The reasons for the failure of a testing apparatus are usually a dirty lens system or a failure of individual components of the recognition electronics.
With the method according to the state of the art, a large number of test bottles had to be used to test the reliability for example of an empty-bottle inspector, every test bottle having a single defect, e.g. a defective closure thread or a foreign body in a single recognition zone. Every type of defect and every recognition zone thus required its own specially prepared test bottle. If a test bottle had revealed several defects, the fact that this bottle was singled out would not have ensured that all the defects were recognized. A set of test bottles therefore consists of e.g. some 10 to 15 bottles.